Wireless telecommunication systems utilize analog and digital transmission techniques. The use of digital methods offers several advantages over analog methods, including: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of message signals, improved security of communication through the use of encryption, and increased capacity. In view of these advantages, digital transmission techniques are growing in popularity.
The growing prevalence of digital wireless telecommunication devices such as telephones, personal digital assistants, personal computers, and other wireless devices is resulting in difficulties for individuals that use hearing aids. When an individual with a hearing aid uses or is close to another individual using a digital wireless telecommunication device, the hearing aid picks up and amplifies the digital signals associated with the device. For example, the hearing aid may pick-up the amplitude modulation inherent in the on-off gated transmitter output signals produced by a wireless telephone using the time division multiple access (TDMA) standard. In a code division multiple access (CDMA) system, transmitter output signals are gated on-off as a function of voice activity. In either system, the gating of the transmitter output signals causes the hearing aid to produce annoying interference in the form of a low frequency buzz accompanied by harmonics and inter-modulation. In some hearing aids, the problem cannot even be eliminated by turning down the volume control. Thus, there are ongoing efforts to reduce the impact that digital wireless telecommunication devices have on individuals using hearing aids.
The difficulties that individuals with hearing aids have experienced as a result of the emerging prevalence of digital wireless telecommunication devices is somewhat offset by the development of Assistive Listening Devices (ALDs). ALDs are designed to reduce the ill-effects of noise, distance, and reverberation on hearing. ALDs consist of a transmitter that is used in a large room or building and a group of receivers that are held by individuals. Typically, the receivers can be connected to a hearing aid to improve sound reception.
The Americans with Disabilities Act (ADA) has specified conditions wherein ALDs are to be used. Businesses are currently installing ALDs to comply with the ADA. In addition, many consumers are purchasing personal ALDs for their homes and offices. Also, more and more organizations, such as churches, that are not covered by the ADA, are still responding to the needs of their constituents by installing ALD technology. ALDs are implemented in a number of ways. Presently, frequency modulation, infrared, and audio induction technologies are prevalent.
Since ALDs are commonly installed in locations where there are a relatively large number of individuals wearing hearing aids, the problem of interference caused by digital wireless telecommunication devices is especially problematic in these locations. Consequently, it would be highly desirable to develop a technique, operative in the presence of assistive listening devices, that reduces hearing aid interference caused by digital wireless telecommunication devices.